The invention generally pertains to a method for improving measurement accuracy in a navigator echo technique used in magnetic resonance (MR) imaging. More particularly, the invention pertains to a method of such type for improving the accuracy for measuring the absolute positional displacement in navigator navigator profiles when using a truncation window, to remove undesirable MR signal from peripheral regions of the navigator profile. As is well known by those of skill in the art, the navigator echo technique is commonly used in connection with respiratory gating, to reduce respiratory motion artifacts in MR imaging, especially in the MR imaging of the heart. In such applications, imaging is directed to anatomic structure such as the liver and lungs, which move with a patient's diaphragm during the respiratory cycle. In the navigator gating technique, a navigator pulse is applied prior to the image data acquisition sequence, to acquire a navigator echo. The acquired echo is then used to determine the displacement of the diaphragm at the time of data acquisition. This is achieved by employing a selected algorithm to compare the acquired navigator echo with a reference navigator echo, obtained prior to data acquisition as part of a set of preliminary data. The reference navigator echo is associated with a specified reference position of a patient's diaphragm, which generally moves periodically along the superior/interior direction during successive respiratory cycles. If the diaphragm is found to lie within a particular displacement window with respect to a reference position, at the time of data acquisition, the acquired MR image data will be accepted. Otherwise, the data will be rejected. Thus, the navigator gating technique comprises an "accept or reject" system. By using only accepted data in image reconstruction, artifacts caused by respiratory motion can be significantly reduced in MR coronary artery imaging or in abdominal MR imaging. The reference position may be selected as that position that corresponds to a particular phase of the respiratory cycle. The preferred position is at end-expiration as it is the most reproducible and consistent phase of the respiratory cycle, i.e., there is usually minimal variation between diaphragm displacement positions between end-expiration phases in successive respiratory cycles.
In the past, different algorithms have been used in connection with echo gating, such as auto-correlation and least-squares algorithms, to determine displacement of the diaphragm as a function of time. However, while these algorithms have been effective in reducing respiratory artifacts in coronary imaging, they typically require substantial computation effort. Frequently, the level of required effort has exceeded the capability of available computational platforms found on certain currently used MR scanning systems. Accordingly, as an alternative, a linear phase-shift algorithm has been developed by T. K. F. Foo, one of the inventors herein, and K. F. King for the measurement of diaphragm position This is set forth in U.S. patent application Ser. No. 08/980,192, filed Nov. 26, 1997 by Foo T K F and King K F. Such application is commonly assigned herewith to the General Electric Company, and is entitled "Fast method for detection and tracking of reference position changes in magnetic resonance imaging". This algorithm, which is now used in MR scanners for prospective navigator gating in 3-D MR coronary angiography, has been found to provide a ten-fold improvement in computational efficiency, and is further described, for example, in an article by Foo, et al entitled "Three-dimensional double-oblique coronary artery MR imaging using real-time respiratory navigator and linear phase-shift processing", Proc. of ISMRM, p. 323 (1998), and in another article by Foo, et al., entitled "An Efficient Method for Calculating Displacement in MR Navigators Using Linear Phase Shifts", Radiology 1997; vol.205 (P), page 213.
In navigator gating using the linear phase-shift algorithm, an acquired navigator echo is inverse Fourier transformed to the frequency domain, to provide a corresponding navigator profile. The navigator profile in the frequency domain is then truncated, to remove undesirable signal from peripheral regions of the navigator profile. More specifically, the navigator echo has an acquisition field of view (FOV) which typically is on the order of 10-20 centimeters. The navigator profile is truncated in the frequency domain by applying a truncation window thereto, which has the effect of reducing the acquisition field of view. The truncated navigator profile is then Fourier transformed back to the time domain. The relative position of the navigator profile can be determined from the linear phase shift of the time domain representation of the truncated navigator profile. One method to determine the linear phase shift in an efficient manner is a method proposed by Ahn, et al., "A new phase correction method in NMR imaging based on autocorrelation and histogram analysis.", IEEE Trans. Med. Imaging, 1987: MI-6: 32-36. By comparing the linear phase shifts between any two profiles, the relative displacement between the two profiles can be measured. The linear phase shift of any one profile corresponds to a relative position of the measured object, this being the top of the right hemi-diaphragm if the respiratory motion is to be monitored.
The algorithm proposed by Foo and King has the advantage of not requiring a reference profile to calculate the positional displacement. As the linear phase shift provides a measure corresponding to that of the position of the diaphragm, for example, the end-expiration position can either be that where the linear phase shift is at maximum or minimum, depending on patient orientation and the direction of the applied read out gradient of the navigator echo MR experiment.
Notwithstanding the benefits of the linear phase-shift algorithm technique described above, it has been found that FOV truncation of a navigator profile, as required by such technique, introduces another source of measurement error. The technique described above may have accurate measures of relative position but poor measures of absolute position. Absolute positional measures are needed when more elaborate motion correction schemes are employed rather than a simple "accept/reject" scheme. One possible use of absolute positional displacement measures is to apply a displacement correction to the acquired MR data, thus allowing a higher efficiency of acquisition by being able to acquire MR data at a greater range of respiratory phases rather than just at end-expiration. One example of such a scheme, known as adaptive navigator echo correction, is described in an article by McConnel et al., "Prospective adaptive navigator correction for breath-hold MR coronary angiography." Magn. Reson. Med. 1997; 37: 148-152.
In an ideal case, when the acquired navigator profile is the same for each acquisition, the difference between the linear phase shifts of any two profiles represents the displacement between the two profiles. However, a spatially fixed FOV truncation window and motion of the diaphragm may cause the navigator profiles to become different. This is especially so if blood vessels or other heterogeneous structures in the liver move into and out of the truncation FOV window, introducing new structures in the navigator profile and hence, altering the linear phase shift of the navigator profile. Using different or dissimilar navigator profiles i result in the undesired measurement error in the linear phase shift. This poses a potential problem if the absolute displacement from a reference position, sometimes taken as the mean end-expiration position, is to be calculated.